1. Field of the Invention
The present invention relates to a water-based drilling mud comprising a minimum of constituents, including a known biopolymer obtained by fermentation, known as scleroglucan, which contains, in particular, the cellular residues of the producing organism.
2. Description of the Related Arts
Drilling mud is a more or less complex mixture most often based on a water-based or oil-based colloidal suspension used for drilling wells. This mud, injected into the succession of drill pipes, circulates by means of an ascending movement in the annular space enclosed between the walls of the geological formations drilled and the succession of pipes. It has multiple functions: in addition to cooling and lubricating without corroding the drilling tool, the mud must be able to raise to the surface the cuttings stripped from the formations. For this purpose it is necessary for the mud to be sufficiently viscous and for its lift to be such that the cuttings is able to remain in suspension in the mud when the flow rate of the latter is zero. The density of the mud must be sufficiently high so as to equilibrate the pressure in the pores of the formations and thus to prevent the formation fluids invading the well. However, this density must not be too high so as not to penalise the progress of drilling, or to fracture the formations.
Another function of the mud is to maintain the wall of the well such that there is no collapse. This function is usually ensured by virtue of the formation of a deposit of a skin of clay particles contained in the mud, this deposit being termed "cake" and covering the wall of the well. The quality of the cake is dependent on the components of the mud and must be suited to the nature of the terrains drilled and of their reactivity. A good quality cake not only permits the wall to be maintained but also permits a filtration control such that the ingress of water and/or mud into the formations is limited. However, plugging of the formations and in particular of the productive formations must be avoided.
A good quality mud must also be inert with respect to the formations which it passes through, in particular if the latter contain salts or reactive clays. Too high a reactivity of the components of the mud with the components of the formations in fact gives rise to reductions in the quality of the wall of the well and a pollution of the mud such that its physical characteristics, such as, for example, its viscosity, are modified, with the consequence that it is no longer able to perform its functions.
The multiple functions of the mud, some of which are antinomic, are the reason why a large variety of muds currently exists. These muds are usually highly complex mixtures of from two to ten or more constituents, the composition of which changes, becoming more complex as drilling progresses, and the use of which necessitates the involvement of specialists. The role of the specialist is to define the mud composition at the start of drilling and to monitor the physical and physico-chemical properties of the mud during operation in order to maintain the said properties and/or to adapt them permanently to the drilling conditions by adding suitable compounds, such as agents for increasing the weight, filtration-reducing agents, viscosity agents, thinners, plugging slurry or lubricants, anti-fermenting agents, neutralising agents and other specific or chemical compounds.
Inorganic colloids increase the viscosities and gelling. Bentonite, for example, which is often used in muds based on fresh water, also contributes to the formation of a cake protecting the walls of the well against collapse and assists in controlling the filtrate.
Among the organic colloids, some are used as filtration-reducing agents, others as viscosity agents and yet others as thinners. The introduction of thinners into the mud composition is necessary when the viscosities increase substantially under the effect of an increase in the clay loading, for example.
Carboxymethylcellulose (CMC), for example, which is a colloid derived from cellulose, is commonly used as additive when it is necessary to improve the carrying capacity of the mud by increasing the apparent viscosity. An improvement in the carrying capacity by means of such a procedure is offset by an increase in pressure loss. Cellulose derivatives, amongst other compounds, may also be present in the composition of the muds as fluid loss reducers.
Electrolytes or other chemical compounds may also be introduced into the composition of the mud, in order to modify the pH, such as sodium hydroxide for example, or in order to neutralise the reactions between the components of the mud and those of the formations.
It is well known that biopolymers may be present in the composition of a mud as a viscosity agent. Apart from remarkable viscosity-improving properties, the biopolymers customarily used for drilling muds have a good compatibility with filtering agents and are also biodegradable, which has the effect of reducing the problems of clogging and removal of pollutant waste.
The quoted document U.S. Pat. No. 4,561,985 describes drilling muds in which either a water-soluble nonionic polysaccharide, selected from the family of cellulose derivatives or guar derivatives, or a water-soluble anionic polysaccharide, selected from the group of carboxymethylcelluloses or the group of polysaccharides obtained from Xanthomonas campestris, or a mixture of these polysaccharides is incorporated as the viscosity agent.
Xanthan in particular is an anionic heteropolysaccharide obtained by fermentation from the action of a microorganism of the Xanthomonas type. Its molecular weight exceeds one million and it is currently very often used in the composition of drilling muds. This hydrophilic colloid not only has the property of thickening but also of stabilising the water-based systems and conferring on them a particularly high viscosity of the order of 1.4 to 1.6 Pa.s measured at 30 rev/min on a Brookfield rheometer for a solution containing 10% by weight of xanthan, in the presence of 5 g/l of NaCl. In addition it has low sensitivity to significant variations in temperature, pH or salinity.
However, the introduction of xanthan into the composition of a drilling mud as the viscosity agent does not enable the other constituents usually needed in the mud composition in order to ensure its other functions to be dispensed with.
Scleroglucan is another polysaccharide produced by fermentation of a fungus of the Sclerotium type, some applications of which are known in the petroleum field. Thus, the quoted document EP 0216661A1 describes an application of scleroglucan in the field of assisted recovery of petroleum. The quoted document FR 2,570,754 cites other possible applications of this product in the field of the treatment of hydrocarbon wells, in particular as completion fluid or cleaning agent for oil or gas transport pipes or as fracturing fluid.
Some properties of scleroglucan are known. Thus, scleroglucan, like xanthan, is a hydrophilic colloid which has the property of thickening and stabilising water-based systems by conferring on them a particularly high viscosity, higher than that obtained in the case of xanthan for identical concentrations of active compounds, and in particular a high independence of these viscosities with respect to variations in pH and temperature within a very wide range, and also rheological properties of the pseudoplastic type. Thus, the viscosity of a scleroglucan solution is virtually independent of the pH between pH 1 and pH 12.5 and of the temperature up to a limit which is located in the vicinity of 130.degree. C. With scleroglucan the viscosity is also highly independent of the salinity.
The xanthans or scleroglucans customarily used in the applications in the petroleum field are refined varieties which are in the form of a white powder obtained by alcoholic precipitation of a fermentation broth after filtration in order to remove the residues of the producing organism. It is customary in the known applications of biopolymers in the field of well treatments to purify or refine the liquid reaction mixture resulting from the fermentation and containing the biopolymer in question so as to separate off all or some of the residues which it contains, by filtering or centrifuging or concentrating the said reaction mixture, so that the treatment agents do not contain more than 10 to 15% of the said residues, consisting of mycelium in the case of scleroglucan.
Xanthan is produced by a bacterium which has a size of only a few nanometers. The fibrous and crosslinked scleroglucan mycelium is of macroscopic size. Even after grinding, the cellular residues remain of the order of one millimeter.